The Dynamic Nature of Brønsted Acid Sites in Cu–Zeolites During NO_x Selective Catalytic Reduction: Quantification by Gas-Phase Ammonia Titration

Abstract

Brønsted acid sites on Cu-exchanged zeolites can be titrated selectively using gaseous ammonia when NH₃ saturation steps are followed by protocols that remove Lewis acid-bound and physisorbed NH₃, such as purging in flowing wet helium at 433 K. NH₃ titrates all H⁺ sites on small-pore chabazite zeolites (SSZ-13) and leads to the complete disappearance of infrared stretches for Brønsted acidic OH groups after saturation (433 K), in contrast with larger n-propylamine titrants that access only a small fraction (<0.25) of H⁺ sites on SSZ-13 under conditions sufficient to titrate all H⁺ sites on medium-pore ZSM-5 zeolites (323 K, 2 h). NH₃ titration of the residual H⁺ sites present in Cu-exchanged SSZ-13 samples (Si/Al = 4.5, Cu/Al = 0–0.20) after oxidative treatments detects two fewer H⁺ sites per exchanged Cu²⁺ ion, as expected to maintain framework charge neutrality. NH₃ titrants detect only one fewer H⁺ site (per Cu) after Cu-SSZ-13 samples undergo a reductive treatment in flowing NO and NH₃ (473 K), however, indicating that each Cu²⁺ cation reduces to form a Cu⁺ and H⁺ site pair. In the context of low temperature (473 K) selective catalytic reduction (SCR) on high aluminum Cu-SSZ-13, we discuss the different mechanistic roles of residual H⁺ sites that remain after Cu²⁺ exchange, whose primary function appears to be NH₃ storage, and of proximal H⁺ sites that are generated in situ upon Cu²⁺ reduction, whose role is to stabilize reactive NH₄ ⁺ intermediates involved in the standard SCR oxidation half-cycle. We highlight how gaseous NH₃ titrants can selectively count H⁺ sites on small-pore, Cu-exchanged zeolites and, in doing so, enable probing the dynamic nature of active sites and catalytic surfaces during SCR redox cycles.